DNS of separating, low Reynolds number flow in a turbine cascade with incoming wakes (original) (raw)
Related papers
2002
The flow around a low-pressure turbine rotor blade with incoming periodic wakes is computed by means of DNS and LES. The latter adopts a dynamic sub-grid-scale model. The computed results are compared with time-averaged and instantaneous measured quantities. The simulations reveal the presence of elongated flow structures, stemming from the incoming wake vorticity, which interact with the pressure side boundary layer. As the wake approaches the upstream half of the suction side, its vortical structures are stretched and align with the main flow, resulting in an impingement at virtually zero angle of attack. Periodically, in the absence of impinging wakes, the laminar suction side boundary layer separates in the adverse pressure gradient region. Flow in the laminar separation bubble is found to undergo transition via a Kelvin-Helmholtz instability. Subsequent impingement of the wake inhibits separation and thus promotes boundary layer reattachment. LES provides a fair reproduction of the DNS results both in terms of instantaneous, phase-averaged, and time-averaged flow fields with a considerable reduction in computational effort.
International Journal of Heat and Fluid Flow, 2020
This paper investigates the vortex dynamics in the suction-side boundary layer on an aero-engine low pressure turbine blade at two different Reynolds numbers at which short and long laminar separation bubbles occur. Different vortical patterns are observed and investigated through large eddy simulation (LES). The results show that at the higher Reynolds number, streamwise streaks exist upstream of separation line. These streaks initiate spanwise undulation in the form of vortex tubes, which roll-up and shed from the shear layer due to the Kelvin-Helmholtz instability. The vortex tubes alternately pair together and eventually distort and break down to small-scale turbulence structures near the mean reattachment location and convect into a fully turbulent boundary layer. At the lower Reynolds number, streamwise streaks are strong and the separated flow is unable to reattach to the blade surface immediately after transition to turbulence. Therefore, bursting of short bubbles into long bubbles can occur, and vortex tubes have larger diameters and cover a part of the blade span. In this case vortex pairing does not occur and vortex shedding process is promoted mainly by flapping phenomenon. Moreover, the results of dynamic mode decomposition (DMD) analysis show a breathing motion as a source of unsteadiness in the separation location, which is accompanied by the flapping phenomenon.
Numerical Simulation of Wake Passing on Turbine Cascades
41st Aerospace Sciences Meeting and Exhibit, 2003
The numerical simulations for two sets of unsteady wake/blade interaction experiments are performed. The first experiment considered was conducted by Kaszeta et al. (2001) to study the effects of periodically passing wakes on transition and separation in a low pressure turbine passage. The second set of experiments considered are the linear turbine cascade experiments of Schobeiri and Pappu (1997) which were conducted to investigate the unsteady boundary layer behavior on a NASA Space Shuttle Main Engine (SSME) turbine blade cascade. In both experiments, moving cylindrical rods are used to mimic the periodically passing wakes upstream of the flows. The numerical simulations are performed for both sets of experiments using a recently developed multi-block Navier-Stokes solver which is capable of handling complex geometries, overset, and moving grids while featuring MPI. Results of fully turbulent computations are presented along with the experimental data. Although comparison with experiment indicates that the general features of unsteady wake/blade interactions are captured with a fully turbulent simulation, it is evident that without taking flow transition into consideration, the flow separation near the trailing edge of the blade can not be predicted.
Effects of Upstream Wakes on the Boundary Layer Over a Low-Pressure Turbine Blade
Journal of turbomachinery, 2022
In the present work, the evolution of the boundary layer over a low-pressure turbine blade is studied using direct numerical simulations, with the aim of investigating the unsteady flow field induced by the rotor-stator interaction. The freestream flow is characterized by the high level of freestream turbulence and periodically impinging wakes. As in the experiments, the wakes are shed by moving bars modeling the rotor blades and placed upstream of the turbine blades. To include the presence of the wake without employing an ad-hoc model, we simulate both the moving bars and the stationary blades in their respective frames of reference and the coupling of the two domains is done through appropriate boundary conditions. The presence of the wake mainly affects the development of the boundary layer on the suction side of the blade. In particular, the flow separation in the rear part of the blade is suppressed. Moreover, the presence of the wake introduces alternating regions in the streamwise direction of high-and low-velocity fluctuations inside the boundary layer. These fluctuations are responsible for significant variations of the shear stress. The analysis of the velocity fields allows the characterization of the streaky structures forced in the boundary layer by turbulence carried by upstream wakes. The breakdown events are observed once positive streamwise velocity fluctuations reach the end of the blade. Both the fluctuations induced by the migration of the wake in the blade passage and the presence of the streaks contribute to high values of the disturbance velocity inside the boundary layer with respect to a steady inflow case. The amplification of the boundary layer disturbances associated with different spanwise wavenumbers has been computed. It was found that the migration of the wake in the blade passage stands for the most part of the perturbations with zero spanwise wavenumber. The non-zero wavenumbers are found to be amplified in the rear part of the blade at the boundary between the lowand high-speed regions associated with the wakes.
2000
An experimental study of the effect of wake disturbance frequency on the secondary flow vortices in a two-dimensional linear cascade is presented. The flow Reynolds numbers, based on exit velocity and suction side surface length were 25,000, 50,000 and 85,000. Secondary flow was visualized by injecting smoke into the boundary layer and illuminating it with a laser light sheet located at the exit of the cascade.
Aerospace Science and Technology, 1998
The flow in a transonic turbine rotor cascade is investigated by both experimental and numerical methods. Measurements include pressure profiles on the blade, total pressure profiles in the blade vane, boundary-layer and wake profiles. Computations are performed by two different solvers with different turbulence models and three different transition models. Results indicate that the introduction of transition models is necessary
International Journal of Rotating Machinery
Incompressible large eddy simulation and direct numerical simulation of a low-pressure turbine atRe=5.18×104and1.48×105with discrete incoming wakes are analyzed to identify the turbulent kinetic energy generation mechanism outside of the blade boundary layer. The results highlight the growth of turbulent kinetic energy at the bow apex of the wake and correlate it to the stress-strain tensors relative orientation. The production rate is analytically split according to the principal axes, and then terms are computed by using the simulation results. The analysis of the turbulent kinetic energy is followed both along the discrete incoming wakes and in the stationary frame of reference. Both direct numerical and large eddy simulation concur in identifying the same production mechanism that is driven by both a growth of strain rate in the wake, first, followed by the growth of turbulent shear stress after. The peak of turbulent kinetic energy diffuses and can eventually reach the suction ...
Journal of Turbomachinery, 2007
The paper experimentally studies the effects of periodic unsteady wake flow and Reynolds number on boundary layer development, separation, reattachment, and the intermittency behavior along the suction surface of a low pressure turbine blade. Extensive unsteady boundary layer experiments were carried out at Reynolds numbers of 110,000 and 150,000 based on suction surface length and exit velocity. One steady and two different unsteady inlet flow conditions with the corresponding passing frequencies, wake velocities, and turbulence intensities were investigated. The analysis of the experimental data reveals details of boundary layer separation dynamics which is essential for understanding the physics of the separation phenomenon under periodic unsteady wake flow and different Reynolds numbers. To provide a complete picture of the transition process and separation dynamics, extensive intermittency analysis was conducted. Ensemble-averaged maximum and minimum intermittency functions wer...
Coherent Structures Formation During Wake-Boundary Layer Interaction on a LP Turbine Blade
Flow, Turbulence and Combustion, 2016
Particle Image Velocimetry (PIV) measurements have been analyzed in order to characterize the dynamics of coherent structures (eddies and streaks) within the suction side boundary layer of a low pressure turbine cascade perturbed by impinging wakes. To this end, the instantaneous flow fields at low Reynolds number and elevated free-stream turbulence intensity level (simulating the real condition of the blade row within the engine) were investigated in two orthogonal planes (a blade-to-blade and a wall-parallel plane). Proper Orthogonal Decomposition (POD) has been employed to filter the instantaneous flow maps allowing a better visualization of the structures involved in the transition process of the boundary layer. For the unsteady case properly selected POD modes have been also used to sort the instantaneous PIV images in the wake passage period. This procedure allows computing phase-averaged data and visualizing structures size and intensity in the different parts of the boundary layer during the different wake passage phases. The contributions to the whole shear stress due to the largest spanwise oriented scales at the leading and trailing boundaries of the wake-jet structures and those associated with streaky structures observed in the bulk of the wake are discussed. Instantaneous images in the wall-parallel plane are filtered with POD and they allow us to further highlight the occurrence of low and high speed traveling streaks (Klebanoff mode). The periodic advection along the suction side of the high turbulent content regions carried by the wakes anticipates both formation and sinuous instability of the streaks inside the boundary layer as compared with the steady case. The dynamics driving the breakdown of the streaks and the consequent formation of nuclei with high wall-normal vorticity have been found to be almost the same in the steady and the unsteady cases. Auto-correlation of the instantaneous images are also presented in order to
The Transport of Vortices Through a Turbine Cascade
Journal of Turbomachinery, 1996
An experiment was conducted to determine how incident vortices created by upstream blade rows interacted with a downstream turbine cascade. Specifically, the kinematics of the vortex transport through turbine blade passages was investigated. A stationary water table and a flow visualization system using the pH indicator Bromothymol Blue was used to visualize the vortices generated by vortex generators placed upstream of a turbine blade cascade. Two test series were conducted. In the first test series, stationary vortex generators were positioned at various locations along the turbine blade pitch to observe how a steady incident streamwise vortex was transported through the turbine cascade. Observations showed an unsteady vortex response of the streamwise vortex when the incident vortex was located at the stagnation area of the blade. In the second test series, the vortex generators were moved to simulate the relative motion of an upstream blade row. In these tests, the unsteady vort...